for me personally the framing via which I understand parts/alters becoming more like 'introjects' over time is just, like. kinning. i already have a framework of how and why it happens that seems to have deviated from how people understand kinning these days but about 6 years ago what you're describing would mostly just be immediately understood as that's what's going on.

identity wise it makes sense. if you have a weak sense of identity sometimes a substitute one will do, especially if you can lean on it without having to use mental energy you might not have currently. it's also possible that they're just not very elaborated and need some kind of rebuilt identity to 'fill in the cracks' so they can more easily advocate for their feelings/needs and be heard, especially if you are more likely to be able to 'hear' or 'spot' them if you've trained yourself to look for fictives. this isn't saying this is a conscious choice, just to be clear. this is more so just what I feel is an unconscious behavior that happens in the background without anyone's intentional decisions.

tldr: if you don't have the outline of a personality, storebought is fine